US 2014/0072098 A1 discloses an x-ray system comprising an x-ray source for emitting x-ray radiation and a radiation detector for detecting the x-ray radiation after having traversed a subject to be examined. The x-ray system further comprises a monitor detector irradiated by radiation emitted by the x-ray source, which has not traversed the subject, wherein the monitor detector is configured as an energy-resolving detector to provide energy-resolved current dose measurement data representing a current dose of the x-ray radiation.
For generating spectral computed tomography projection data a spectral computed tomography system comprises a radiation device which provides polychromatic radiation traversing a subject to be imaged and a detector which detects the polychromatic radiation after having traversed the subject. The spectral computed tomography system is adapted to generate the spectral computed tomography projection data for different projection directions and to reconstruct a computed tomography image based on the spectral computed tomography projection data.
The spectral properties of the radiation device may vary over time due to, for instance, a roughening of an anode of an x-ray tube of the radiation device, unintended variations of an applied tube voltage, unintended focal spot movements, et cetera. The variations of the spectral properties of the radiation device can result in artifacts in the reconstructed computed tomography image.